Inductive Power Transfer (IPT) systems are, well known. An example of one such system is shown generally in FIG. 1. Such systems are also described comprehensively in the prior art, including for example U.S. Pat. No. 5,293,308.
In recent times IPT systems have been used in electric vehicle battery charging applications. A significant advantage of IPT for electric vehicle battery charging is its tolerance to misalignment between the primary magnetic structure and the magnetic structure of the secondary (also referred to in this document as a pick-up) apparatus. As shown in FIG. 1, an IPT charger typically includes a switching power supply such as a resonant converter which is supplied from a utility grid and in turn provides an alternating current to a primary inductor which may comprise a track or a magnetic structure in the form of a pad for example. The varying magnetic field which is provided by the primary pad is then intercepted by one or more secondary magnetic structures which usually comprise a further pad or coil, represented in FIG. 1 by inductance L2. The power received is conditioned by a resonant network and power controller in the pick-up apparatus and then supplied to the electrical load, for example a battery, being charged.
Physical movement or displacement between the ferrimagnetic material and coils in the pick-up (L2) relative to the primary pad (L1) necessarily introduces variations in the magnetic coupling between the pads and also introduces variations in the pad self inductance. Also, variations in the tuning generally in IPT systems can change dependent on other factors, for example component tolerances and variations over time (e.g. tuning capacitor degradation), breakage of ferrite in magnetic structures, etc. Therefore, it is impossible for both the primary and secondary charging pads to always be accurately tuned over given range of movement within a specified power transfer zone without adopting self-tuning circuitry. In a fixed frequency primary side current controlled system, this places additional reactive load on the power supply, although it does not affect the power transfer capability of the pick-up providing the track current can be regulated at a desired magnitude. However, the mistuned resonant network introduces additional reactive load in the system and this reactive load increases the losses in the system
in general, including the power supply, additional switch conduction loss and losses in the magnetic coupling structures. Therefore, creating systems in which the reactive load due to mistuning is minimised is advantageous.